1. Field of the Invention
The present invention relates to a brake system for an engine-operated vehicle and, more particularly, it relates to a brake system of the described type, adapted for being cooperated with an engine mounted on the vehicle and provided with a unit capable of employing intake negative pressure occurring in the engine in order to boost a brake operating force that is applied to the brake system of the vehicle. The present invention also relates to a method of controlling an operation of a brake system.
2. Description of the Related Art
Japanese Laid-open (Kokai) Patent Application Publication No. 2005-163634 discloses that in a vehicle provided with: a variable valve mechanism capable of continuously changing a lift characteristic of an intake valve; a negative pressure adjusting valve capable of adjusting intake negative pressure of an engine; and a brake booster into which intake negative pressure occurring in the engine may be introduced, whereby the intake negative pressure of the engine is increased as soon as negative pressure prevailing in the brake booster is reduced to a pressure level which is less than or equal to a predetermined value.
Japanese Laid-open (Kokai) Patent Application Publication No. 2006-168412 discloses that assistant fluid pressure is added to master cylinder fluid pressure based on a deviation between an actual vehicle deceleration and a target vehicle deceleration that corresponds to master cylinder fluid pressure.
Moreover, Japanese Laid-open (Kokai) Patent Application Publication No. 2006-168412 discloses that assistant fluid pressure is added to master cylinder fluid pressure so that a brake operating force may be boosted, even after a boosting limit point of a brake booster, in a way similar to that before the boosting limit point.
In the invention disclosed in the first-mentioned Japanese Laid-open (Kokai) Patent Application Publication No. 2005-163634, when the negative pressure within the brake booster decreases, negative pressure is increased regardless of the presence of a request for brake application or of a request for an increase in a braking force. As a result, there has been a problem in that pumping loss is significant, resulting in reduced fuel economy.
At this stage, it is to be understood that throughout the description of the present patent application, a reduction in negative pressure refers to a technical condition such that a pressure below atmospheric pressure becomes close to atmospheric pressure.
Furthermore, the invention disclosed in Japanese Laid-open (Kokai) Patent Application Publication No. 2006-168412 is to compensate for a deficiency of wheel cylinder pressure with fluid pressure supplied from a pump, and it is not intended to suppress a reduction in intake negative pressure of an engine. Hence, the invention of this publication has nothing to do with suppression of deterioration in fuel economy that must be caused by pumping loss.